I think having a large percentage of the user list that doesn't post is inevitable, and not always a negative. I'd be interested in seeing how many of the infrequently posting users still visit regularly. I bet a lot of people sign on, search and read threads until they find the solution to their problem, and log out without interacting. Some people just aren't interested in posting, but still benefit through troubleshooting or just reading for entertainment. I'm a member of ADVRider, for example, but rarely post. I do a lot of research on there, and often just look at the pretty pictures without posting much. I'm also a member on a couple Ford Ranger forums just because I had to register to get access to some of the information. I'm not a Ranger enthusiast, so I don't have any interest in being part of the Ranger/Explorer community. It's just a truck to me. I show appreciation where necessary, though.
Then there's the load of people who buy a bike, do some Googling and end up here, register, and forget about it. It might take a little more effort, but you could own an XJ600 that you ride, maintain, modify, etc without ever visiting this site. Especially if you've owned other bikes and have a lot of general moto knowledge. To a lot of people, it's just a bike the way a car is just a car. Probably also plenty who buy a bike, learn, and move on to a different one without ever really getting beyond basic non-model-specific issues.
But I get what you mean about wanting more interaction. That was kind of my idea behind the ride challenges. We're still picking up new frequent posters/core members, though, even if at a slow rate. They're always good for reigniting some discussion or providing new project/ride report threads to follow.